1. Field of the Invention
The present invention relates to a current source apparatus, a light-emitting-device drive apparatus and a digital-analog converting apparatus, which are used in electronic devices and integrated circuits.
2. Description of the Prior Art
A current source apparatus for use in an electronic device and an integrated circuit, which has a current mirror structure, is disclosed as first prior art in “Semiconductor Circuit Design Technology” (Nikkei BP Inc., first edition, sixth printing, p. 302-303).
FIG. 28 shows a circuit diagram of the current source apparatus according to the first prior art. In FIG. 28, symbols “M1” and “M2” denote MOS transistors constituting a current mirror. A symbol “CM1” denotes the input-side circuit of the current mirror. A symbol “CM2” denotes the output-side circuit of the current mirror. A symbol “1” denotes a supply-voltage application terminal through which a supply voltage is applied to the input-side circuit CM1 and the output-side circuit CM2 of the current mirror. A symbol “2” denotes the current input terminal of the input-side circuit CM1 of the current mirror. A symbol “3” denotes the output terminal of the input-side circuit CM1 of the current mirror. A symbol “4” denotes the input terminal of the output-side circuit CM2 of the current mirror. The input terminal 4 of the current mirror output-side circuit CM2 is connected to the output terminal 3 of the current mirror input-side circuit CM1. A symbol “5” denotes the current output terminal of the current mirror output-side circuit CM2. A symbol “6” denotes a control-signal input terminal to which a signal for current interruption control is input. A symbol “CS” denotes a current source connected between the current input terminal 2 of the current mirror input-side circuit CM1 and the ground. A symbol “Iin” denotes the current that flows in the current source CS. A symbol “SW4” denotes a switch inserted between the MOS transistor M1 and the current input terminal 2. A symbol “INV1” denotes an inverter logic circuit connected between the control-signal input terminal 6 and a control terminal of the switch SW4. A symbol “LD” denotes a load connected between the current output terminal 5 of the current mirror output-side circuit CM2 and the ground. A symbol “Iout” denotes the current that flows across the load LD.
In the current source apparatus, the MOS transistors M1 and M2 constitute the current mirror, and the inverter logic circuit INV1 works on the switch SW4 according to the control terminal input signal to be applied to the control-signal input terminal 6. When the switch SW4 is ON, the input current Iin is input to the MOS transistor M1. As a result, the current is inverted by the current mirror, so that the output current Iout is output to the load LD. When the switch SW4 is OFF, the input current Iin is not input to the MOS transistor M1. Therefore, the output current Iout does not flow to the load LD from the current mirror (Iout=0).
A current source apparatus and a digital-analog converting apparatus for use in an electronic device and an integrated circuit are disclosed as second prior art in Japanese Patent Laid-Open Publication No. H11-251912 (page 7 and FIG. 1).
FIG. 31 shows a circuit diagram of the current source apparatus according to the second prior art. In FIG. 31, a symbol “SW5” denotes a switch connected between the common source and the common gate of the MOS transistors M1 and M2. The switch SW5 is used in place of the switch SW4 in FIG. 28. The other structure is the same as that of the current source apparatus in FIG. 28.
In the current source apparatus, the MOS transistors M1 and M2 constitute the current mirror, and the inverter logic circuit INV1 works on the switch SW5 according to the control terminal input signal to be applied to the control-signal input terminal 6. When the switch SW5 is OFF, the input current Iin is input to the MOS transistor M1. As a result, the current is inverted by the current mirror, so that the output current Iout is output to the load LD. When the switch SW5 is ON, the input current Iin flows through the switch SW5 and does not flow into the MOS transistor M1. Therefore, the output current Iout is not supplied to the load LD from the MOS transistor M2.
Another current source apparatus and a light-emitting-device drive apparatus, which have been conventionally used in an electronic device and an integrated circuit, are disclosed as third prior art in Japanese Patent Laid-Open Publication No. 2003-188465 (page 10 and FIG. 1).
FIG. 34 shows a circuit diagram of the current source apparatus according to the third prior art. In FIG. 34, a symbol “INV3” denotes an inverter logic circuit. A symbol “C2” denotes a capacitor. This current source apparatus differs from the current source apparatus in FIG. 28 in the addition of the inverter logic circuit INV3 and the capacitor C2. The other structure is the same as that of the current source apparatus in FIG. 28.
In the current source apparatus, the MOS transistors M1 and M2 constitute the current mirror, and the inverter logic circuit INV1 works on the switch SW4 according to the control terminal input signal to be applied to the control-signal input terminal 6. When the switch SW4 is ON, the input current Iin is input to the MOS transistor M1. As a result, the current is inverted by the current mirror, so that the output current Iout is output to the load LD. Further, the output of the inverter logic circuit INV3 is added to the output current Iout via the capacitor C2. When the switch SW4 is OFF, the input current Iin does not flow into the MOS transistor M1. Therefore, the output current Iout is not supplied to the load LD from the MOS transistor M2. Further, the output of the inverter logic circuit INV3 positively turns the MOS transistor M1 off via the capacitor C2.
The conventional digital-analog converting apparatus for use in an electronic device is demanded of being able to operate fast and linearly changing the amount of the output current when it is in operation.
At the time of emitting light, the conventional light emitting device to be used in an electronic device is demanded of being able to switch between the light-ON state and the light-OFF state at a high speed and linearly changing the amount of light emission.
At the time of driving a light emitting device, the conventional current source apparatus and light-emitting-device drive apparatus, which are used in such a light emitting device, are demanded of being able to can interrupt the output current at a high speed, and linearly changing the amount of the current.
At the time of driving a load, the conventional current source apparatus to be used in a digital-analog converting apparatus is demanded of being able to interrupt the output current at a high speed, and linearly changing the amount of the current.
With the current source apparatus as shown in FIG. 28 in use, for example, even when the switch SW4 is turned off, the charges charged in the gate capacitors of the MOS transistors M1 and M2 cannot be pulled out. When the switch SW4 is turned off, therefore, the load current cannot be interrupted immediately. The load current thus has a held waveform as shown in FIG. 29, which shows the waveforms of the control terminal input signal to be applied to the control-signal input terminal 6 and the load current flowing across the load LD. Accordingly, the current source apparatus cannot interrupt the current at a high speed. In addition, the linearity of the output current is lost in a high-speed operation, as shown in FIG. 30, which shows the relationship between the input current Iin and an output current effective value or an output current root-mean-square value Ioutrms for the target value, a low-speed mode and a high-speed mode. It is apparent that a light-emitting-device drive apparatus constructed by using the current source apparatus does not become an adequate light-emitting-device drive apparatus. The current source apparatus, when used in a digital-analog converting apparatus, causes an error in converted output.
With a current source apparatus shown in FIG. 31 in use, for example, even when the switch SW5 is turned off, it takes time to charge the gate capacitors of the MOS transistors M1 and M2 thereafter, so that the current cannot rise soon. Therefore, the load current would have a waveform with a slow rise as shown in FIG. 32, which shows the waveforms of the control terminal input signal to be applied to the control-signal input terminal 6 and the load current flowing across the load LD. Accordingly, the current source apparatus cannot interrupt the current at a high speed. In addition, the linearity of the output current is lost in a high-speed operation, as shown in FIG. 33, which shows the relationship between the input current Iin and the output current root-mean-square value Ioutrms for the target value, a low-speed mode and a high-speed mode. It is apparent that a light-emitting-device drive apparatus constructed by using the current source apparatus does not become an adequate light-emitting-device drive apparatus. The current source apparatus, when used in a digital-analog converting apparatus, causes an error in converted output.
With the current source apparatus as shown in FIG. 34 in use, for example, the load LD is driven by the ON/OFF action of the switch SW4 and via a capacitor C2. The time and the current amount that are compensated for by the capacitor C2 are determined by the capacitance C2, the gate capacitances of the MOS transistors M1 and M2, and the source resistance of the MOS transistor M1. Therefore, the amount of the load current cannot be controlled by the current interruption by the switch SW4. The load current would therefore have a waveform in which an uncontrollable current amount is added to the proper current amount, as shown in FIG. 35 which shows the waveforms of the control terminal input signal to be applied to the control-signal input terminal 6 and the load current flowing across the load LD. What is more, the linearity of the output current is lost in a high-speed operation, as shown in FIG. 36, which shows the relationship between the input current Iin and the output current root-mean-square value Ioutrms for the target value, a low-speed mode and a high-speed mode. It is apparent that a light-emitting-device drive apparatus constructed by using the current source apparatus does not become an adequate light-emitting-device drive apparatus. The current source apparatus, when used in a digital-analog converting apparatus, causes an error in converted output.